1. Field of the Invention
The present invention relates to electrical engineering, and more specifically concerns microswitches.
2. Description of the Prior Art
The accuracy in operation of automatic lines and production control systems depends to a great extent on the sensitivity of microswitches used therein, which microswitches must be accurate in transmission of information. Errors caused by the microswitches during their service are difficult to eliminate practically.
The movable contacts in the microswitches are actuated preferably with the aid of metal plates capable of responding to the changes in temperature or pressure, which plates when being tensed or compressed undergo only a slight bending. Therefore, one of the main characteristics of a microswitch is the sensitivity thereof determined by the length of the movement path of the actuating link (the shorter is the travel path required for operating a movable contact element the higher is the sensitivity of the microswitch), and hence by the quantity of energy required for such microswitches to operate, that is to make or break contacts. An important requirement imposed on microswitches is that they must provide reliable and trouble-free switching under conditions wherein they are exposed to vibrations and shocks even at a low speed of movement of the actuating link. This requirement is determined by the contact resistance which depends on the contact pressure, which contact pressure also determines the resistance of a microswitch to vibration and shocks arising during operation.
U.S. Pat. No. 1,098,074 discloses a microswitch featuring a high sensitivity. This microswitch comprises an insulating base 1' (see FIG. 1), stationary contacts 2', 3' secured on said base 1', a movable contact 4', and a three-link system 5' of levers adapted for selectively changing the position of the movable contact 4'. The three-link system 5' of levers comprises an actuating link 6', an intermediate link 7', and a contact link 8', said links being connected therebetween in series. The actuating link 6' and the contact link 8' are fastened on the insulating base 1' for rocking, with the movable contact 4' fastened on the contact link 8', and one of said links, for instance intermediate link 7' being made elastic.
When the microswitch is in its initial position, the intermediate link 7' which is preliminarily tensed acts on the end of the contact link 8' with a force P. A normal force P.sub.1 to produce a contact pressure is P.sub.1 =P.multidot. sin .alpha., where .alpha. is an angle between the contact link 8' and the intermediate link 7'.
Under the action of an external force F' the actuating link 6' is caused to displace and the intermediate link 7' changes its position relative the contact link 8', in which case the angle .alpha. decreases as a result of which the contact pressure P.sub.1 respectively decreases. When the actuating link 6' reaches the position of direct operation of the microswitch (that is, when point A reaches line I--I which is a line of unstable initial state of the contact link 8', to take a position A.sub.1), the angle .alpha. and the contact pressure P.sub.1 are equal to "0" (zero).
Shown in FIG. 2 is a contact pressure graph showing a contact pressure variation depending on the movement path of the actuating link 6', where 1 is the path described by a point A of the actuating link 6'.
As the actuating link 6' moves further and the point A intersects the line I--I, the contact 4' is caused to switch over at its natural speed of motion, in which case the actuating link 6' with the point A may reach a position of overtravel (position A.sub.2).
When the external force F' does not any more act on the actuating link 6' the latter under the action of the spring moves so that the point A reaches the line II--II, which is a line of an unstable changed position of the contact link 8' (position A.sub.3).
Differential travel L.sub.A of the actuating link 6' at the point A is equal to the distance between the points A.sub.1, A.sub.3, determined from the relationship ##EQU1## where H is a contact gap between the contacts 4', 3'; L is a distance from the axis "O" of rotation of the contact link 8' to the axis of the contacts 2', 3', and at the same time is a length of the actuating link 6'; .DELTA.L is a displacement of the point A from the axis O, which is necessary to provide a snap actuation of the contact 4', and wherefrom ##EQU2##
The differential travel of the actuating link 6' at the point to which the external force F' is applied will be ##EQU3## where L.sub.1 is a length of the actuating link 6' measured from its axis of rotation to the force F' point.
Taking into account small values ##EQU4## and H, it is practically possible to obtain a differential motion which is equal L.sub.F' =0.05 to 0.001 mm.
As may be seen from the above description of the prior art microswitch operation the time of the direct and reverse switching of the movable contact 4' does not practically depend on the position and speed of movement of the actuating link 6'. However, the contact pressure in such sensitive microswitches changes with the displacement of the actuating link 6' at its speed of motion, from a nominal value to a minimum one, and may even be equal to zero when the actuating link 6' is in a position close to the position in which the microswitch operates (see FIG. 2).
At low speeds of motion of the actuating link 6' of the microswitch a long time during which the contacts are closed, with the contact pressure being insufficient, may cause contact burning, melting, and even their sticking to one another.
In order to provide trouble-free operation of the prior art microswitches the speed of motion of the actuating link 6' of the microswitch must exceed 5 mm/s.
If the speed of motion of the actuating link 6' is lower than 5 mm/s (like in limit switches or pressure and temperature sensors), actuating mechanisms are used to operate movable contacts, which actuating mechanisms comprise a four-link lever system and are adapted to provide a contact operation time and contact pressure which do not practically depend on the position of the actuating link before the microswitch operates, and hence on the speed of movement of said actuating link.
There is known a microswitch disclosed in USSR Inventor's Certificate No. 752,528, (Int. Cl. H 01 H 13/26) which comprises an insulating base 1" (see FIG. 3). and fastened thereon stationary contacts 2", 3", a movable contact 4", and a four-link lever system 5". The four-link lever system 5" is a chain including an actuating link 6", two middle links 7", 8" one of which being an intermediate link and the other one being a contact link, and a support link 9". The actuating and support links 6", 9" are end links and are secured on the insulating base 1" for rocking, and one of the middle links, namely contact link 8" carries a movable contact 4" alternately interacting with the stationary contacts 2" and 3". The microswitch also includes a limit stop 10" fastened on the insulating base and adapted to hold one of the middle links 7" and 8" in the end positions in the direction of displacement of the movable contact 4", and a lead 11" also secured on the insulating base 1" and electrically connected with the movable contact 4".
The intermediate link 7" is made elastic.
When the microswitch is in its initial position, the preliminarily tensed intermediate link 7" applies a force P to the end of the contact link 8" butting against the stop 10". A force P.sub.3 constituting a contact pressure is P.sub.3 =P.sub.2 sin .beta.=P cos .alpha. sin .beta., where .alpha. is an angle between the contact link 8" and the intermediate link 7", .beta. is an angle between the contact link 8" and the support link 9".
Under the action of the external force F" the actuating link 6" is caused to displace, the intermediate link 7" is tensed and changes its position relative the contact link 8", in which case the angle .alpha. decreases, the angle .beta. remains constant and the contact pressure due to the tension of the intermediate link 7" increases.
When the actuating link 6" is forced to the position of the direct operation (that is, when the point A reaches the line I--I of the unstable state of the contact link 8" and takes position A.sub.1) the angle .alpha. is equal to zero and a contact pressure P.sub.3 '=P.sub.1 ' sin .beta..
Shown in FIG. 4 is a contact pressure graph showing variation of the contact pressure as a result of the actuating link 6" displacement, where 1 is a path of the point A of the actuating link.
As the actuating link 6" moves further and the point A intersects the line I--I, the contact 4" is switched over moving at its natural speed. In this case point A of the actuating link 6" may reach the position A.sub.2.
When the external force F" is not any more applied to the actuating link 6" the latter under the action of the return spring moves so that its point A reaches the line II--II which is a line of unstable changed position of the contact link 8", i.e. it reaches position A.sub.3 which is the position of reverse operation of the microswitch.
To ensure switching-over of the contact 4", the distance between the elements of the stop 10" restraining the motion of the actuating link 6" should be somewhat longer than 2H.
In order to simplify calculation let us assume that the gap between the restraining elements in the stop 10" is 2H. The differential travel of the actuating link 6" at point A is equal to the distance between points A.sub.1 and A.sub.3, which distance is determined by the following equation. EQU L.sub.A =H+2.DELTA.h,
where H is a contact gap between the contacts 2" and 4".
Since .DELTA.BCD" is similar to .DELTA.O"EA.sub.1 (FIG. 3) the length of the path .DELTA.h is determined from the relationship ##EQU5## where .DELTA.L is displacement of point A from the axis O", which is necessary to provide snap operation of the contact 4";
L is a distance from the rotation axis O" of the contact link 8" to the stop 10", and at the same time is a length of the actuating link 6".
The differential travel L.sub.A will be ##EQU6##
As may be seen from the above description the differential travel of the point A on the actuating link 6" of the microswitch shown in FIG. 3 consists of two components: a value H equal to 1 to 1.5 mm, and a value ##EQU7## which is three times that of the differential travel of the actuating link 6' at point A in the prior art microswitch (FIG. 1), with the same values H, .DELTA.L, L.
Thus, the microswitch shown in FIG. 3 provides reliable switching only at a low speed of motion of the actuating link under condition wherein the switch is exposed to vibration and shocks occurring during operation. However, the sensitivity which is determined by the travel path of the actuating link displacement (differential travel) is not sufficiently high.